Image sensors are integrated circuit devices that include a plurality of sensor elements, or pixels, formed in a semiconductor substrate. The sensor elements are used for sensing a volume of exposed light projected towards the semiconductor substrate. As with many types of integrated circuits, a desire exists to advance image sensors as semiconductor technology develops. A common development in semiconductor technology is the adoption of copper in one or more conductive layers. For example, interconnect metal in advanced semiconductor technology uses copper, which add various etch stop layers and barrier layers between inter-metal dielectric layers to prevent the copper from migrating to nearby silicon oxide dielectric material. Silicon nitride has been widely used as barrier layer and stop layer. However, different indexes of refraction between silicon nitride and silicon oxide will cause optical interference, and this will result in a modification or degradation of the image sensor's performance.